Restriction fragment length polymorphism (RFLP) analysis was performed in three Finnish families with different subtypes of acute intermittent porphyria (AIP): 1) cross-reacting immunological material (CRIM)-negative with low erythrocyte porphobilinogen (PBG)-deaminase activity, 2) CRIM-positive with low PBG-deaminase activity and 3) CRIM-negative with normal PBG-deaminase activity. The disease-associated RFLP haplotype (A2B1C2) of the PBG-deaminase gene was the same in each family. In all three families, RFLP linkage analysis resulted in highly positive lod scores. The maximal lod score (4.3) was obtained at the recombinant fraction of zero, thus confirming a tight linkage of AIP to the PBG-deaminase locus. Of the 62 family members tested, 30 had the disease-associated haplotype; in 5 of them, conventional tests for AIP were normal and in one, uncertain. RFLP analysis can thus reveal new gene carriers and help in the diagnosis of individuals with uncertain results in other laboratory tests.

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